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<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <meta http-equiv="X-UA-Compatible" content="ie=edge" />
    <title>Document</title>
  </head>

  <body>
    <script>
      // 父类有加法方法
      class Father {
        constructor(x, y) {
          this.x = x
          this.y = y
        }
        sum() {
          console.log(this.x + this.y)
        }
      }
      // 子类继承父类加法方法 同时 扩展减法方法
      class Son extends Father {
        constructor(x, y) {
          // 利用super 调用父类的构造函数
          // super 必须在子类this之前调用
          super(x, y)
          this.x = x
          this.y = y
        }
        subtract() {
          console.log(this.x - this.y)
        }
      }
      var son = new Son(5, 3)
      son.subtract() // 2
      son.sum() // 8
    </script>
  </body>
</html>
